What is bird gravel?

Bird grit or _gravel_ is small stones or sand ingested by many birds to aid digestion. Grit remains in the gizzard (a muscular stomach), where ingested food such as seeds is ground down by the constant contractions of the gizzard's muscular walls, and the grit's abrasive action. Grit does not dissolve anywhere in the digestive tract; undigested grit is eventually passed out of the body with feces.

Most poultry have a crop for storing food before entering the gizzard, but pigeons, doves, sandgrouse and flamingos do not. Seed-eaters that do not have crops often swallow larger grit to temporarily act as a crop, in which food can soften before entering the gizzard. Sandgrouse store water in their plumage, which they later release for their chicks and for moistening hard seeds stored in the crop; the grit then helps to break up the seeds in the gizzard.